linux-kernel - Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking

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Date:   Mon, 8 Aug 2022 15:25:34 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Hao Luo <haoluo@...gle.com>, Youlin Li <liulin063@...il.com>
Cc:     ast@...nel.org, john.fastabend@...il.com, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        kpsingh@...nel.org, sdf@...gle.com, jolsa@...nel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking

On 7/30/22 12:48 AM, Hao Luo wrote:
> On Fri, Jul 29, 2022 at 3:43 PM Youlin Li <liulin063@...il.com> wrote:
>>
>> In adjust_scalar_min_max_vals(), let 32bit bounds learn from 64bit bounds
>> to get more tight bounds tracking. Similar operation can be found in
>> reg_set_min_max().
>>
>> Also, we can now fold reg_bounds_sync() into zext_32_to_64().
>>
>> Before:
>>
>>      func#0 @0
>>      0: R1=ctx(off=0,imm=0) R10=fp0
>>      0: (b7) r0 = 0                        ; R0_w=0
>>      1: (b7) r1 = 0                        ; R1_w=0
>>      2: (87) r1 = -r1                      ; R1_w=scalar()
>>      3: (87) r1 = -r1                      ; R1_w=scalar()
>>      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
>>      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0xffffffff))  <--- [*]
>>      6: (95) exit
>>
>> It can be seen that even if the 64bit bounds is clear here, the 32bit
>> bounds is still in the state of 'UNKNOWN'.
>>
>> After:
>>
>>      func#0 @0
>>      0: R1=ctx(off=0,imm=0) R10=fp0
>>      0: (b7) r0 = 0                        ; R0_w=0
>>      1: (b7) r1 = 0                        ; R1_w=0
>>      2: (87) r1 = -r1                      ; R1_w=scalar()
>>      3: (87) r1 = -r1                      ; R1_w=scalar()
>>      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
>>      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0x3))  <--- [*]
>>      6: (95) exit
>>
>> Signed-off-by: Youlin Li <liulin063@...il.com>
> 
> Looks good to me. Thanks Youlin.
> 
> Acked-by: Hao Luo <haoluo@...gle.com>

Thanks Youlin! Looks like the patch breaks CI [0] e.g.:

   #142/p bounds check after truncation of non-boundary-crossing range FAIL
   Failed to load prog 'Permission denied'!
   invalid access to map value, value_size=8 off=16777215 size=1
   R0 max value is outside of the allowed memory range
   verification time 296 usec
   stack depth 8
   processed 15 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

Please take a look. Also it would be great to add a test_verifier selftest to
assert above case from commit log against future changes.

Thanks,
Daniel

   [0] https://github.com/kernel-patches/bpf/runs/7696324041?check_suite_focus=true